Yes. You can code your entire UI with your preferred .NET language.
No. Instead of providing a drag and drop designer, the Avalonia IDE extensions support a live previewer, which will render a preview of the UI, refreshing as your modify the XAML.
You can use a community project to enable hot reload with Avalonia.
Yes. You can compile for macOS and Linux from Windows. You'll likely need to package your app on those platforms to create release packages of your app.
Yes. You can develop for Android today and we have a preview demonstrating the beginnings of iOS support. You should however give careful consideration to each platform and ensure that your app behaves well on smaller, touch focused devices.
No. We have an experimental proof-of-concept that demonstrates Avalonia running in the browser using Wasm but it's not considered production ready. We're waiting on upstream improvements before continuing with this work.
Check out the community guide to see how you can get involved with the project.
Debian 9 (Stretch)+
macOS High Sierra 10.13+